1. Field of the Invention
The present invention relates to a linear motion rolling guide unit which is applied to sliding portions of machine tools, various precision machining equipment and testing equipment.
2. Description of the Prior Art
The linear motion rolling guide unit has been in use for carrying and guiding a relatively heavy equipment back and forth over a relatively long distance with high precision. An example conventional linear motion rolling guide unit is shown in FIG. 6.
FIG. 6 shows a perspective view of a conventional linear motion rolling guide unit. As shown in FIG. 6, the linear motion rolling guide unit has two parallel track rails 2 securely mounted on a base 11 such as bed or mount; two or more sliders 1 mounted astride on each of the track rails 2; and a table 4 securely mounted on these sliders 1 with bolts 33. Equipment placed on the table 4 are moved back and forth along the track rails in the direction of arrow R. In such a conventional linear motion rolling guide unit, the track rails 2 are each formed with raceway surfaces 5 extending longitudinally on both side walls 21 thereof and the sliders 1 are slidably mounted astride each of the track rails 2. Each of the slider 1 is slidable relative to the rail 2 and consists of a casing 3 having raceway surfaces at positions facing the raceway surfaces 5, a number of rolling elements such as balls or cylindrical rolls, which are trapped between the opposing raceway surfaces to allow relative motion between the rail and the casing, and end caps 6 attached to the longitudinal ends of the casing 3.
The slider 1 is freely slidable on the track rail 2 because of the rolling elements that circulate along the raceway surfaces 5 of the track rail 2. The rolling elements in a load region, i.e., those traveling on the raceway surface 5 of the track rail 2 are led into a direction changing path, which is formed in the end cap 6, and further into a return path formed parallel to the raceway surface in the upper part of the casing 3, so that the rolling elements run in an endless circulating path. The rolling of the rolling elements trapped under load between the raceway surfaces of the slider 1 and the raceway surfaces 5 of the track rail 2 permits the slider 1 to be slid on the track rail 2.
A galvanic corrosion prevention bearing is disclosed in the Japanese Utility Model Laid-Open No. 85016/1990. This ball-and-roller bearing has both or one of its inner and outer races coated over the outer surface with an insulating film. That is, the race to be coated with the insulating film is formed with an annular groove along the circumferential surface, into which an insulating film is installed. Further, the center of the annular groove is shifted from the center of the bearing so as to reliably fix the insulating film to the race, preventing dislocation or floating of the insulating film.
Another example of the galvanic corrosion prevention bearing is disclosed in the Japanese Utility Model Laid-Open No. 121222/1991. In this bearing, the insulating coating over the surface of the race is formed of polyphenylene sulfide resin, which contains glass fibers, so that the coating has an excellent water absorption, heat resistance and strength, thus providing a stable anti-galvanic corrosion performance.
Still another example of the galvanic corrosion prevention bearing is disclosed in the Japanese Patent Laid-Open No. 4311/1992. In this ball-and-roller bearing, the outer diameter surface and end surface of the outer race are covered with a fiber reinforced composite material.
A further example of the galvanic corrosion prevention bearing is disclosed in the Japanese Utility Model Laid-Open No. 119615/1991. This ball-and-roller bearing consists of a pair of bearings with the external diameter surfaces and the outer side surfaces of each outer race formed with an insulating coating and with the facing inner side surfaces thermally conducted to each other.
As described above, the technical philosophy of coating the race with an insulating material is already known in the ball bearings. In the linear motion rolling guide units, however, a technique has yet to be developed for electrically insulating the members. In the linear motion rolling guide unit as shown in FIG. 6, where sliders 1 move back and forth on the track rails 2, when a current flows through the track rails 2 and the sliders 1, a galvanic corrosion occurs between the track rails 2 and other members, i.e., base 11 and sliders 1, and between the sliders 1 and other members, i.e., table 4 and track rails 2, degrading the precision of the mounting surfaces or sliding surfaces of these members as well as machining precision and durability of machine tools.